Hemihydrate

Hemihydrate refers to a chemical substance in which half of the water of crystallization of a hydrate remains in the solid. In mineralogical and chemical contexts, the term most often applies to calcium sulfate hemihydrate with formula CaSO4·0.5H2O, also known as bassanite in its mineral form and widely known commercially as plaster of Paris when produced as a powder.

Production and structure: Hemihydrate is produced by calcination of calcium sulfate dihydrate (gypsum) CaSO4·2H2O at about 120–170°C, removing 1.5 moles of water per mole of calcium sulfate. The resulting hemihydrate can crystallize in two polymorphs, alpha (α-hemihydrate) and beta (β-hemihydrate); bassanite is the natural mineral form of this compound.

Applications: Hemihydrate is widely used as plaster of Paris in construction, medicine, dentistry, sculpture, and mold making. When mixed with water, it rehydrates to the dihydrate and hardens as a rigid mass; the setting is exothermic and the material gains strength as the crystal network forms.

Other hemihydrates: The term can be applied to other salts that form hydrates with half a mole of water, but calcium sulfate hemihydrate is the most common and commercially important example.